Okay, I've read way too many Deku/Kaminari fics, and the tension usually comes from playing with their core dynamic. Deku's anxiety meets Kaminari's apparent carelessness, and writers squeeze that gap for all it's worth.
A classic move is the 'misunderstood sacrifice'. Denki does something reckless to protect Izuku, maybe taking a hit or short-circuiting himself to overload a circuit and save him, but Izuku interprets it as Denki not taking things seriously or not valuing his own safety. That creates this brittle, aching distance where Deku is worried and angry, and Denki feels unappreciated and stupid, neither communicating properly. The tension isn't just 'will they kiss?', it's 'will they even manage to talk to each other again without one of them shutting down?'
Another layer is the intellectual/instinct divide. Deku overthinks every interaction, analyzing Denki's smiles for hidden meaning. Denki, meanwhile, operates on gut feeling and impulse. That mismatch means Deku might be quietly falling apart over a casual touch Denki doesn't even remember giving. The slow burn there is brutal because one character is miles ahead emotionally while the other is blissfully unaware, building this huge potential energy.
The best fics I've seen use Denki's 'wheys' moments not just for comedy, but as a genuine point of insecurity. Post-overuse, when he's out of it, Deku has to care for him, and that vulnerability—Denki confused and trusting, Deku tender but guilty for maybe causing it—creates a super intimate, charged atmosphere. It flips their usual dynamic completely.